1. Field of the Invention
The present invention relates to a display unit employing an image forming optical system for forming an image on a transparent screen according to an image provided by an indicator, and particularly, to a head-up display type unit for displaying an image over a background in a field of view.
Head-up displays for displaying indications of various instruments over a front view (a background) are conventionally used in aircraft, particularly combat planes, and recently have become frequently used in automobiles, to improve safety.
Such head-up displays would prove useful and convenient to reduce fatigue and improve efficiency if applicable to a clerk's window in a banks, etc., to speech or lecture tables, and to sportscaster's seats, because the head-up display can display an image (second information) from an indicator over a front view (first information) so that a clerk at the window, a lecturer, or a sportscaster can keep his or her eyes on the first information (such as a customer, an audience, or a game) and simultaneously observe the second information (such as account information, lecture materials, and data of athletes) without moving his or her eyes each time.
The present invention has been developed to meet the above requirements. Note, the present invention is not limited to the above applications but is applicable to any kind of display units, such as display units of personal computers, word processors, and other office automation equipment.
2. Description of the Related Art
Display units of personal computers and word processors are, for example, CRTs, liquid crystal display units, and plasma display units, and these conventional display units have a plate-like or box-like shape and are opaque, and thus a user cannot look at something else while closely observing the screen of the display unit. If the user wants to look at information displayed on the display unit while looking at something else, the user must move his or her eyes toward the display unit and away from the thing at which the user is looking. If the display unit is transparent, however the user can see both a background and an image on the display unit without moving his or her eyes.
One technique of overlaying a background and a displayed image one upon another is the head-up display (HUD). This technique forms an image (a virtual image) provided by an indicator in a field of view of a user, and the virtual image is displayed as at a distance, and thus the user is not required to move his or her eyes or the focal points of the eyes (FIG. 29). In FIG. 29, a background image and an image from an indicator 105 through a transmission-type hologram 103 overlap on an image combiner 101. The image combiner 101 is usually a reflection-type hologram, because a reflection-type hologram has wavelength selectivity which can improve the efficiency of use of both the indicator light and the background light. The head-up display itself is a known technique, and therefore, is not explained in more detail.
The head-up display technique is naturally applicable for the transparent display unit mentioned above, but is not easily applied because a virtual image (an indication) is not formed on the surface of a device (the surface of an image combiner). A conventional display unit (such as a CRT and a liquid crystal display) cannot be employed, simply because it appears incongruous and because the surface of the image combiner must be brought too close to the eyes of the user when an image is displayed at a least distance of distinct vision (e.g., about 30 cm).
These problems will be explained with reference to FIG. 29. The HUD comprises the indicator 105, transmission-type hologram 103, and reflection-type hologram (image combiner) 101. An image provided by the indicator 105 forms a virtual image Iv through the two holograms 103 and 101 on the opposite side of the image combiner 101 with respect to the eyes of a user, so that the displayed image (virtual image) Iv and a back ground B overlap. A direction of light diffracted or reflected by a hologram depends on the wavelength of the light incident thereon (wavelength dispersion). When light having a wide wavelength band is used instead of a monochromatic light such as a laser beam, the wavelength dispersion is usually compensated with two holograms. A hologram is originally meaningful when used as an image combiner. To correct any chromatic aberration, a pair of holograms (101 and 103) is used, but it is difficult to form an image forming system having a large screen and a short focal length (a short distance between a virtual image and the image combiner) with two holograms, because this increases a numerical aperture, and thus does little to correct the chromatic aberration. Unlike a conventional display unit (a light emission type), the head-up display cannot align the position of the device (a display screen) with the position of a displayed image, and therefore, to see an image at a least distance of distinct vision (about 30 cm), the image combiner must be very close to the face of a user, which is not practical.
An object of the invention is to realize a display unit which is transparent and having a display screen substantially aligned with the surface of the unit.